Terahertz (THz) radiation is radiation having a frequency on the order of 1012 hertz (Hz) and shows increasingly promise in a variety of applications including medical imaging without harmful radiation, computing, observing quantum phenomena, and security. A known method of THz generation is by optical down-conversion in nonlinear optical materials using a laser-pulse to create a time-dependent polarization that radiates an electric field. The efficiency of THz generation in optical down-conversion processes, however, is low even in the most efficient of the current systems. To being with, optical-to-THz conversion efficiency is taught to be limited by the Manley-Rowe photon conversion limit. The Manley-Rowe limit relies upon an assumption that the number of generated THz photons cannot exceed the number of pump optical photons. In traditional schemes for generating THz radiation with a frequency of omega1 this limit is expressed by the following basic principles. The traditional schemes often involve introducing an optical beam with a frequency omega3 into a material that interacts with the optical beam (omega3) to generate optical beams having a frequency other than omega3. For example, generation of THz radiation having a frequency of omega1 has been accomplished by the mixing of two optical beams with frequencies omega2 and omega3, so that omega1=omega3−omega2. The Manley-Rowe limit assumes that even if all the photons of the optical beam with a frequency of omega3 are converted to beams with the frequency omega2 and omega1, the energy of the omega1 beam can never exceed the conversion factor related to omega1/omega3.
In addition, typical interactions between generated THz radiation and the optical pulses are short. This short interaction length between THz and optical pulses is a result of phase and group velocity-mismatch between optical and THz waves due to the strong dispersion of nonlinear materials. Optical down-conversion is most efficient in ZnTe crystals, where the coherence length reaches several millimeters. However, group velocity dispersion also leads to broadening of femtosecond optical pulses in ZnTe reducing the peak power and conversion efficiency.
The interaction length between THz and optical pulses can be extended in quasi-phase matching (QPM) structures. The fundamental idea is to exploit the velocity mismatch between the optical and THz pulses in a poled nonlinear crystal to generate a THz waveform, which corresponds to the domain structure of the poled nonlinear crystal.
Newly developed orientation-patterned GaAs (OP-GaAs) has been suggested for improving the effectiveness of THz generation in QPM structures, as taught by U.S. Pat. No. 6,273,949 and U.S. Pat. No. 5,355,247; each of these patent documents is fully incorporated herein by reference. While the conversion efficiency of these methods is increased, these methods still leave room for improvement.
These and other issues have presented challenges to the generation of THz radiation, including improving the efficiency of THz radiation generation.